The present invention is directed to methods and devices for magnetic field measurement and more specifically, detection of magnetic anomalies present in a detection zone.
The field of the invention is vehicle detection devices and systems, and particularly, systems which magnetically detect the presence of a vehicle.
The earth""s magnetic field is normally constant over time, at a given location. When a ferro-magnetic object moves to a specific location, the earth""s magnetic field, at that location, is perturbed. Since most vehicles contain ferro-magnetic material, their presence causes a perturbation in the earth""s magnetic field.
Various sensors have been developed to detect vehicles by measuring the perturbation of the earth""s magnetic field. The prior art indicates that they are used primarily to classify the type of vehicle, and measure the speed, direction of travel, and presence of a moving vehicle. These sensors function by measuring short duration perturbations (AC phenomenon) in the earth""s magnetic field.
The prior art suggests that existing magnetic sensors can be used to detect nonmoving vehicles. However, when a magnetic vehicle sensor is used as part of a control system to control machinery (such as car wash equipment) the existing devices do not adequately protect against false detection or missed detection of vehicles. Reliable detection of non-moving vehicles requires accurate measurement of a steady-state perturbation (DC phenomenon) of the earth""s magnetic field.
To accurately detect the presence of a non-moving vehicle, the absolute magnitude of the earth""s ambient magnetic field must be known. The present invention is a system which includes means for producing a signal that indicates no vehicle is near a sensor. During, or just after, the assertion of this signal, the earth""s ambient magnetic field is measured with the sensor and the resulting value is saved in non-volatile memory. This signal is asserted during the initial start-up, or whenever physical changes are made near the sensor (equipment moved or added, building changes, etc.). Following loss, and restoration of power to the system, the value of the ambient magnetic field is retrieved from the non-volatile memory enabling the system to immediately detect a vehicle without manual intervention.
Another aspect of the invention is to monitor long term changes that may occur in the ambient magnetic field. Measurements are made at regular intervals when vehicles are not present and these measurements are filtered to provide a more accurate measurement of the ambient magnetic field.
Yet another aspect of the present invention is the continuous calibration of the magnetic field sensors.